This post is focused on holypriests. But the findings are probably useful for other specs and classes too.The TLDR version: intellect is vastly superior to all other forms of regen.Of course, that point is moot if you have a double solace or the spark of hope trinket setup. In that case, you don't need regen anymore. But for the rest of us, here is why and by how much.

MP5

1 MP5 costs twice as much itemization as a stat point. This means you can get 2 spirit or 1 MP5 for the same itemization cost. As a consequence, The MP5 stat is really worth 0.5 MP5. This number never scale with anything. It also doesn't hold the secondary benefits that f.ex spirit grants you, with 0.25 spellpower per point of spirit, or int, which grants 1/166th % of crit. For this reason, MP5 is usually considered subpar for holypriests. But at low gear levels, it is the best stat for regen.

When calculating in itemization, 0.5 MP5 is worth 1 itemization points.

Spirit

Next up, spirit. The tried and true formula for regen is known and "simple".

At 1293 intellect, 5 * sqrt(int) * 0.005575 = 1. I often use this level of intellect to simplify the formula. Most priests have more than that in intellect these days though, so this is a low estimate. But let us run with it. The simplified version is thus:MP5 = 5 * (0.001 + sqrt(Int) * Spirit * 0.005575) * 0.60, rounded up

Half of which is FSR regen. Most priests hardly leave the FSR these days, so the following is a good approximation to FSR regen through spirit.MP5 = Spirit * 0.60

Of course, you can add 5% through talents, and another 10% through BoK. You are probably only inside the FSR for only 95% of the time. And you are likely to have more than 1293 intellect. And most importantly, your will have a notable uptime on holy concentration. I've found number between 20 and 50% to be good estimates, but if you have other data to back this up, I'm all ears. At 35% Holyconc uptime, you can add 17.5% to your spirit income. But just for the heck of it, lets go with 50% uptime today, that means 25% gains!FSRMP5 = Spirit * 0.30

That means 0.3 (spirit regen) * 1.05 (SoR) * 1.10 (BoK) * 1.25 (Holyconc) * 1.05 (FSR time, found by solving "X*0.95 + 2X*0.05") * Y (int synergy) = 0.46 * Y (int synergy). By adding high int, spirit gets better, but squarerooted, so it's not amazing. Still, I think we can hit 0.5 by factoring in that.

As an estimate to spirit regen, I often hold the rule of thumb that 1 spirit is worth 0.5 MP5. If you want a perfect estimation of your regen through spirit, I can heartily recommend Zusterke's regen calculator. So I'm going to run with this value that 1 spirit = 0.5 MP5 today.

1 Spirit also offers 0.25 spellpower through Spiritual Guidance, multiplied by 5% from SoR and 10% from BoK. 1 spellpower is worth 0.86 itemization, so this is worth 0.215*1.05*1.10=0.248 itemization points.

Combined, this means 1 spirit is worth 1.248 itemization points.

Intellect

And then to the point of this post. Intellect. It is my claim that intellect outperform spirit badly, even when considering the spellpower gains. I will measure this through itemization.

Intellect grants three sources of regen, and grants a minor boost to critical hit.

- Regen through the spirit formula

- Regen through active cooldowns

- Regen through a large manapool reserve

Let's cover each in detail.

Regen through the spirit formula

Calculating the gains from int in this formula is relatively complex. The gains from 1 int is significantly larger at low intellect levels than the gains from 1 int at high intellect levels.

But... assuming 1293 int is the "baseline" intellect, anything beyond that should be counted as value for intellect, really. But the gains from intellect is not linear. But you can easily calculate the difference, and put that into the formula to see what the gains of that intellect is.

You can do similar calculations for any level of intellect. But you will find that the gains from intellect through the spirit formula is in this ballpark. Of course, higher spirit means significantly higher returns from intellect as well. I will run with this number for the conclusion.Let us consider a case with 1500 spirit and 1500 int. The difference between sqrt(1293) sqrt(1500) is 35.958-38.729 = 2.771. By putting this back into the spirit formula instead of sqrt(int), we can get the gains from the 207 int we are over the baseline int value.

MP5 = 5 * (0.001 + 2.771 * 1500 * 0.005575) * 0.60, rounded up

MP5 = 69.50

By going up 207 int, we gained 69.5 MP5. FSR regen is half of that, so we're at 34.75 MP5. Since we added 207 int in this case, that's 0.16 MP5 per point of intellect.

Ultimately, int gains from the spirit formula depend on your level of spirit. High spirit means int is better. High int means spirit is better. If the spirit formula was the only gains of regen, the optimal balance between int and spirit is a 2spirit:1intellect ratio. Attaining that balance is neither possible nor advisable, because there are other gains from intellect as well which will offset this balance badly. But since you cannot actively avoid gear with spirit on it anyway, I wouldn't worry about this ratio too much, TBH.

- Regen through active cooldowns

This is a hard calculation, since it totally depends on playstyle. Assuming you use your cooldowns at every possible cooldown, you will get the following:

- Replenishment: 0.15 MP5 per point of intellect

- Shadowfiend: 0.125 MP5 per point of intellect (significantly more if used with hymn of hope or bloodlust)

- Hymn of Hope: 0.03 MP5 per point of intellect

- Manatide totem: 0.06 MP5 per totem per point of intellect

- Arcane Torrent: 0.02 MP5 per point of intellect (if blood elf, but let's assume you are not)

That's 0.37 MP5 per point of intellect if added together. However doing so is not really sound math. These cooldowns will not be used on every cooldown. Even replenishment will not see a 100% uptime. But if you use the shadowfiend with hymn of hope (and you should), even the 0.37 MP5 value may be a bit low. But, I will run at 75% overall efficiency of this number. That means 0.28 MP5 per point of intellect from active cooldowns. If you want a more precise simulation, again, Zusterkes calculator will give you an answer.

- Regen through a large manapool reserve

Many people forget about this part. But having a large manapool is equivalent to regeneration by itself. People often forget, because you carry a mindset of wanting to heal forever. Don't think like that. You need to heal for at most 10 minutes before the boss enrages. And all fights lasting longer than 7.5 minutes will have slow periods in them which allows you to regen some mana. Zusterkes otherwise excellent calculator does not cover this income at the time of writing.

With that thought in mind, consider this:A priest with 100 mana and 0 income, spending 2 mana per second, will go OOM in 50 seconds.

A priest with 50 mana and 1 mana per second income, spending 2 mana per second, will also run OOM in 50 seconds.

In this example, having 50 max mana or 1 mana per second is interchangable.

If the fight lasts longer than 2.5 minutes, getting that regen as MP5 is superior. If the fight lasts shorter than 2.5 minutes, getting that regen as intellect is superior. But as we just pointed out, int also gives other sources of regen.We know that 1 int gives 15 max mana. For the same itemization cost, we could get 0.5 MP5.

In a fight lasting 150 seconds, 0.5 MP5 will grant you 15 mana.

150 seconds is 2.5 minutes.

I will assume our fights lasting 7.5 minutes. It's not always true, many fights last longer. Some last considerably shorter. But I think this is a good estimate.

In a fight lasting 7.5 minutes, having 0.5 MP5 is 3 times as good as having 15 max mana. Conversely, 1 int is worth 0.5/3=0.16 MP5 by just having a large manapool!

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Summary for intellect:

1 int is worth ~0.16 MP5 through the spirit formula

1 int is worth ~0.28 MP5 through active cooldowns

1 int is worth ~0.16 MP5 through a large manapool reserve

Summarized, 1 int is worth ~0.6 MP5.

The latter two are turn is increased by 10% through BoK, the first is increased by ~5%, leaving us at ~0.65 MP5 per point of intellect!

This is worth 1.3 itemization points.

In addition, 1 int gives 1/166th % crit. This equals 0.277 crit rating, which in turn costs 0.277 itemization points.

Combined, this means 1 intellect is worth 1.577 itemization points.

TLR summary

Int is the best stat for a holypriest, and the best stat for regen.

When gearing for MP5, you get exactly what you pay for.

When gearing for spirit, you get 24.8% more itemization for free.

When gearing for intellect, you get 57.7% more itemization for free.